Let me answer your questions one by one.
How hard is it to learn Html? It is very simple depending on your willingness to learn. But, really it is very simple.
How long will it take me? It shouldn't take you more than three weeks to understand to a very good length, depending on the time you are willing to put in per day, of course. Like 5hours is ok.
Would I be able to learn it free and if so where? Maybe YouTube? Of course, you will learn it free of charge. Others have pointed you to where to go. But, you should consider w3schools and codecademy.
Is there a market for people wanting HTML websites? Not really, you'll have to combine it with other web technologies. But don't worry yourself about that now. Just note that in other to create a nice dynamic website, you are going to need to know a client side scripting language, server side scripting language, CSS and of course HTML. Although, others are also required to get really wonderful results.
However, avoid worrying yourself about the others right now otherwise you might never get to learn anything. So, take one step at a time. And lastly avoid compiling series of reading materials, they are just absolute waste of time. Get few materials, and focus on them, as time goes on you'd understand all you need to know. Remember practice makes perfect.
.